Many composite structures are fabricated with a core, such as a foam core, sandwiched between a pair of skins which may comprise laminated layers. The cores often are molded by thermoset or thermoplastic compounds which often are not dimensionally controlled or controllable. This is because the coefficient of thermal expansion can be different in three mutually perpendicular axes, and can be large, when compared to isotropic materials of construction, such as metals. A typical epoxy resin used in composite materials may have as much as four times the coefficient of expansion of metal. Fillers and reinforcements can be added to the resin to add dimensional stability. For example, long continuous fibers may be added, such as carbon or glass fibers, to lessen the thermal expansion. Such fibers can be very effective to control the expansion along the length of the fiber but are not very effective in controlling expansion perpendicular to their length.
Attempts have been made to achieve isotropic thermal expansion control, while at the same time reinforcing and strengthening the material, by adding chopped fiber or fillers. This has become somewhat common and examples of such materials are carbon or glass fibers, clay, mica, Teflon, wollastonite, molybdenum disulfide, and a variety of other filler materials. However, such fibers often become oriented in a common plane, yielding low coefficients of thermal expansion in two directions but not the third. Although some degree of random orientation of the reinforcement has been achieved in thermoplastic resins, the result has not been totally satisfactory when forming relatively thin structures. Obviously, the reduction in the coefficient of thermal expansion corresponds to an increased amount of fiber disorientation. Another disadvantage in the use of fibers is that they are relatively heavy when lightweight core materials are desired.
Other attempts to achieve the desired properties in a core material have evolved around the addition of a filler material comprising microspheres. This material reduces the weight of the core but does not possess the dimensional control provided by fibrous material, such as carbon fibers. The use of microspheres, sometimes called microballoons, can present an additional problem in that the microballoons are prone to be crushed under high pressures. This problem is magnified when thermoplastic resins are desired in the composition, whereby the thermoplastic resins are quite amenable to injection molding, compression molding or extrusion, which involves the application of high pressures. Combining microspheres with fibrous materials has been proposed but balancing the parameters involved has not achieved satisfactory results. To achieve the required density, and use fibrous materials, blowing agents may have to be added. The conclusion is that the use of thermoplastic resins has become quite desirable in core compositions, but with the high pressures involved, the dimensional stability desired, and other related parameters and problems encountered, a new and improved composition is needed.
This invention is directed to satisfying the above need and solving the above problems.